Track Eye Gaze with Lines

[enkeli]

I am trying to add lines (aka: "rays", whatever you want to call them) to indicate where an avatar is looking. So ideally the line originates at the avatar's head and extends out according to their gaze. This is what I'm using to create the lines:

		[ x, y, z ] = avatar.getpos()
		y = 1.4
		viz.startlayer( viz.LINES )
		viz.linewidth( 1 )
		gazeOrigin = viz.vertex( x, y, z ) 
		gazeEnd = viz.vertex( x, y, z + 5 ) 
		gaze = viz.endlayer()
		gaze.color( viz.BLUE )
		gaze.alpha( 0.3 )

Easy enough, but now I want to update where the line is pointing when the avatar moves their head. I want to keep the origin vertex stationary while changing the angle of the line. Suggestions?